1. Field of the Invention
The present invention relates, in general, to photonic device manufacturing methods and, in particular, to methods for shifting the bandgap energy of quantum well layers and photonic devices and photonic integrated circuits formed thereby.
2. Description of the Related Art
In the manufacturing of photonic devices, it is often desirable to employ processes that facilitate the monolithic integration of multiple photonic devices on a single substrate. This monolithic integration increases the yield, performance and functionality of the photonic devices and reduces manufacturing cost. The multiple photonic devices can include active photonic devices (e.g., lasers, optical intensity modulators, optical phase modulators, optical switches, optical amplifiers, optical saturable absorbers, optical pulse reconditioners, optical wavelength converters, phosistors [photon transistors], variable optical attenuators, optical detectors) and passive photonic devices (e.g., optical waveguides, optical gratings and optical splitters, optical beam couplers, multi-mode interference devices, optical polarizers, optical polarization beam splitters, optical wavelength filters and optical resonators).
Photonic devices are typically made up of III-V semiconductor materials. Monolithic integration of multiple photonic devices, however, usually requires that photonic devices with different III-V semiconductor material characteristics (e.g., different bandgap energies) be formed on a single substrate. For example, to monolithically integrate a photonic laser device and a passive optical waveguide device, the photonic laser device must contain active III-V semiconductor materials that emit light at a particular lasing wavelength, while the passive optical waveguide device must contain passive III-V semiconductor materials that are transparent to the light emitted from the photonic laser device. Therefore, the emission/absorption wavelength of the III-V semiconductor materials in the photonic laser device must be different from that of the III-V semiconductor materials in the passive optical waveguide device.
The emission/absorption wavelength of III-V semiconductor materials is determined by their bandgap energy. Thus, monolithically integrating multiple photonic devices on a single substrate requires a process for shifting the bandgap energy (and thus the bandgap wavelength) of a selected substrate portion to a value different from that of another substrate portion. Such a process is often referred to as “bandgap engineering.”
One approach to bandgap engineering is called “Quantum Well Intermixing (QWI)”. Photonic devices typically have a quantum well (QW) structure that includes a quantum well layer disposed between (i.e., sandwiched between) two barrier layers. The barrier layer has a larger bandgap energy than the quantum well layer and acts as a potential barrier to confine electrons in the quantum well layer. QWI shifts the bandgap energy of a quantum well layer in selected areas by intermixing (i.e., interdiffusing) the atoms between the quantum well layer and its adjacent barrier layers.
FIGS. 1A and 1B are energy diagrams for a quantum well structure 10 with a single quantum well layer 12 and barrier layers 14 before and after an exemplary QWI process, respectively. Before the QWI process, barrier layers 14 and quantum well layer 12 have abruptly different transition energies, resulting in a square-shaped finite energy potential well (see FIG. 1A). As is well known to those skilled in the art, such a square-shaped finite energy potential well has a quantitized transition energy (labeled Eginit in FIG. 1A) that is dependent on the thickness of the quantum well layer 12.
During the exemplary QWI process, the square-shaped finite energy potential well structure of FIG. 1A is converted to a parabolic-shaped finite energy potential well structure, as shown in FIG. 1B. Because of the conversion from the square shape to the parabolic shape, the effective thickness of the quantum well layer 12 is modified and typically becomes narrower. This results in a shift in the transition energy Eginit to a new transition energy EgQWI, thus providing the desired shift in bandgap energy and bandgap wavelength.
One conventional QWI process is referred to as Impurity Induced Disordering (IID). This process first involves the creation of crystal site vacancies known as point defects. As is well known to those skilled in the art, the atoms in a semiconductor material form a crystal structure and are arranged in a periodic lattice-like fashion. In the case of III-V semiconductor materials, two types of atoms, namely the group III atoms and the group V atoms, are arranged to occupy alternating lattice sites in the crystal structure. These group III and group V atoms exchange electrons and exist as electrically charged ions at the lattice sites. In the case of Aluminum Gallium Arsenide (AlGaAs), the Al and Ga atoms are from group III and the As atom is from group V. In the case of Indium Gallium Arsenide Phosphide (InGaAsP), the In and Ga atoms are from group III and the As and P atoms are from group V. A “crystal site vacancy” is formed when an ion is missing from a lattice site. Such a crystal site vacancy can be formed, for example, by knocking an ion off its site to an “interstitial space” in the crystal structure. A single isolated vacancy or a small group of vacancies is called a “point defect.” A point defect carries the opposite electric charge of the missing ion.
A point defect can move around in the crystal structure when the crystal structure is heated. Heating can cause the atoms in the sample to vibrate violently. Under such thermal vibration, an atom from a lattice site close to the vacancy may move into the vacancy and fill the vacancy (i.e., void). The original site of the atom then forms a new vacancy or point defect, resulting in an effective movement of the point defect from one lattice site to another. Before QWI can happen, point defects are either created at the quantum well structure or migrated to the quantum well structure (for example, to the boundary between a quantum well layer and a barrier layer) via a thermal process.
After the above-mentioned process of having the point defects at the boundary between the quantum well layer and a barrier layer, a subsequent high-temperature crystal annealing step is needed to cause quantum well intermixing (QWI) to occur. Upon high temperature annealing, thermal energy causes some of the point defects in the barrier layer to be filled by atoms from the quantum well layer and the point defects in the quantum well layer to be filled by atoms from the barrier layer. In addition, some of the interstitial atoms with opposite charge will also migrate down to meet with some of the vacancies and heal (i.e., fill) the vacancies. On the other hand, some of the vacancies will migrate deep down to the substrate and become diluted out. In short, this annealing process causes an effective exchange (or “intermixing”) of the atoms between the quantum well layer and the barrier layer(s), resulting in a shift of the bandgap energy to a higher value.
More specifically, in a conventional IID process, the creation of the crystal site vacancies (i.e., point defects) is typically accomplished by introducing impurity atoms/ions into the quantum well structure using a room temperature ion-implantation technique. The ion implantation step is followed by a high temperature anneal step, typically conducted at a temperature of around 800° C. for a GaAs and AlGaAs based quantum well structure, or around 600° C. for an InGaAs and InGaAsP based quantum well structure. In such a conventional IID process, donor ion species (e.g., Si) and acceptor ion species (e.g., Zn) have been utilized. These donor and acceptor ion species are known as shallow-level ion species, because they have a relatively low energy of ionization in III-V material semiconductors. During the high temperature anneal step, some of the point defects created by the implanted atoms/ions, the interstitial ions (i.e., those ions knocked from their lattice sites) and the implanted atoms/ions, will diffuse into the quantum well layer and barrier layers and promote intermixing (or interdiffusion) between atoms in the quantum well layer and the barrier layers.
The use of high-energy and/or high dose (i.e., a dose of greater than 1×1015 cm−2) ion implantation in conventional IID processes is known, however, to cause severe damage in the quantum well structure. The more severe damage includes crystal defects known as loops, lines, complexes and clusters. In general, these crystal defects are referred to as “complex defects.”
Conventional ion implantation based IID processes have several drawbacks. These drawbacks include difficulty in producing: (i) a low-loss waveguide photonic device due to free carrier absorption from implanted shallow-level ion species and scattering loss from complex defects induced by the IID process; (ii) photonic devices with controlled electrical characteristics (e.g., a desirable electrical conductivity or pn-junction properties) due to the aforementioned free carriers and complex defects, as well as due to re-distribution of dopants during the high temperature anneal step; and (iii) a photonic device with a high quality gain layer due to the IID process-induced complex defects, which create carrier recombination centers resulting in shorter carrier lifetime and lower optical gain.
Active or passive photonic devices such as amplifiers, lasers, detectors, modulators, couplers, transparent waveguides, and many others, require either good electrical conductivity, low waveguide loss, or high optical gain. The conventional ion implantation based IID processes do not adequately produce high quality photonic devices either in the form of single device or integrated multiple devices.
The main criteria needed for a QWI process to achieve high quality photonic devices can be more specifically described as follows:                (i) The QWI process must be capable of producing low loss passive waveguides with losses of lower than 4 dBcm−1.        (ii) The active gain QW structure must not be drastically affected by the QWI process. It is typically desirable to have a gain deterioration after the QWI process of no more than 50% of the original gain value.        (iii) The resolution in the wavelength shift has to be relatively high. A process that can control wavelength shift with a resolution accuracy of better than 10 nm is typically desirable.        (iv) It is desirable to achieve a large enough wavelength shift to produce transparent or low loss passive waveguides. Typically for low loss passive waveguides, it is desirable to have wavelength shift of >100 nm. Another reason for desiring a large wavelength shift is to be able to produce active devices that can operate throughout a large part of the optical communications sub-band. A typical requirement to cover a reasonable part of the optical communications sub-band is to have a wavelength shift of >50 nm.        (v) It is desirable for the process to be capable of producing more than two optical emission/absorption wavelengths on a single substrate (e.g., wafer) so that more than two different types of semiconductor photonic devices can be integrated on a single chip.        (vi) The process must be able to shift the wavelength of QW structures that are placed substantially away from an upper surface, as most of laser and waveguide structures have a relatively thick upper cladding layer. A typical requirement would be the capability of inducing QWI for QW structures that are placed 1.5 μm away from an upper surface.        (vii) Recently, there has been increasing interest in making small photonic integrated devices monolithically on a single wafer. In order to achieve a high-density of integration, the process must be capable of shifting the optical wavelength with high spatial selectivity (i.e., resolution) on a very small area. This spatial resolution requirement will be dependent on the critical dimensions of the devices. For a conventional device, a typical resolution requirement will be 3-5 μm. For photonic devices with submicron feature sizes, such as grating or nano-scale photonic devices, the resolution requirement would be less than 3 μm.        
The conventional IID process cannot adequately achieve the majority of criteria (i)-(vii). For example, conventional IID processes can achieve criteria (iii) and (iv), but have difficulty in satisfying criteria (i), (ii), (v) and (vi).
Still needed in the field, therefore, is a method for shifting the bandgap energy of a quantum well layer without inducing complex defects or generating significant free carriers. In addition, the method should avoid the redistribution of dopants into the quantum well layer and satisfy the majority of criteria (i)-(vii) above.